1. Field of the Invention
This invention relates to the biological treatment of aqueous solutions such as wastewater to remove dissolved cyanides.
2. Description of the Prior Art
Metal complexed cyanides in wastewaters form as a result of interactions of free cyanide with metals present in the wastewater and exhibit varying degrees of stability, toxicity, and treatability. Thiocyanate, a pollutant commonly found in cyanide-containing wastewaters, is formed through the interaction of free cyanide with sulfur-containing species both present in the wastewater.
In certain industrial processes, such as the beneficiation of gold and silver and electroplating, cyanide is an essential reagent. Since free cyanide, complexed cyanides, and thiocyanates are potentially toxic to humans and aquatic organisms, these compounds and complexes must be removed from wastewaters prior to their discharge into surface or ground waters serving as potential potable water sources, marine or fresh water habitats.
Conventional chemical treatment processes utilized in treating these wastewaters include ozonation, alkaline chlorination, and copper-catalyzed hydrogen peroxide. Although ozonation removes free cyanides, metal complexed cyanides, and thiocyanates through oxidation, ozonation will not oxidize the extremely stable iron complexed cyanides (ferri- and ferro-cyanide). Also, the oxidation of cyanides and thiocyanates with ozone produces ammonia which is toxic to both humans and aquatic organisms in elevated concentrations. Further treatment is required to remove residual cyanides and ammonia. Ozonation also requires expensive and sophisticated equipment which is difficult to maintain and operate without highly skilled technicians.
Alkaline chlorination removes the compounds removed by ozonation, as well as removing metals through precipitation at elevated pH. Unfortunately, the process effluents contain residual iron complexed cyanides, chlorine, and ammonia, which are toxic and must be removed through further treatment. Excessive chlorine quantities are required when thiocyanate is present, and the efficiency of metals removal is highly pH dependent.
Copper-catalyzed hydrogen peroxide (see U.S. Pat. No. 3,617,567 to Mathre) removes free and complexed cyanides (including the stable iron complexed cyanides) through oxidation, but does not remove thiocyanate or by-product ammonia, both of which are toxicants which must be removed through further treatment. Also, the copper which must be added as a catalyst is toxic to aquatic organisms and must be removed prior to discharge of the wastewater. A relatively large quantity of expensive hydrogen peroxide must be utilized regardless of the cyanide concentration.
Various biological treatment methods have been proposed for removing cyanide from wastewaters. U.S. Pat. No. 3,756,947 to Fujii et al. discloses the addition of a microorganism selected from the genera Alcaligenes and/or Achromobacter to an activated sludge to degrade nitriles and cyanides. U.S. Pat. No. 3,940,332 to Kato et al. discloses the use of a microorganism selected from the genus Norcardia to remove nitriles and cyanides from wastewaters. U.S. Pat. No. 3,660,278 to Mimura suggests the acclimatization of a microorganism from the genus Pseudomona to decompose hydrogen cyanide. Multiple stage chemical and biological wastewater treatment processes are disclosed in U.S. Pat. No. 3,816,306 to Roy and U.S. Pat. No. 4,188,289 to Besik.
The bacterial strain Pseudomonas paucimobilis is described in Holmes, et. al. (1977) Intl. J. Sys. Bacteriol. 27:133-146.